Expert Witnesses: Hard and Soft Sciences

Rule 702. Testimony by Expert Witnesses 
A witness who is qualified as an expert by knowledge, skill, experience, training, or education may testify in the form of an opinion or otherwise if the expert’s scientific, technical, or other specialized knowledge will help the trier of fact to understand the evidence or to determine a fact in issue.

Rule 703. Bases of an Expert’s Opinion Testimony 
An expert may base an opinion on facts or data in the case that the expert has been made aware of, reviewed, or personally observed. If experts in the particular field would reasonably rely on those kinds of facts or data in forming an opinion on the subject, they need not be admissible for the opinion to be admitted.

Rule 704. Opinion on an Ultimate Issue 
An opinion is not objectionable just because it embraces an ultimate issue. 

Rule 705. Disclosing the Underlying Facts or Data and Examining an Expert About Them
(a) Stating an Opinion Without Disclosing the Underlying Facts or Data. Unless the court orders otherwise, an expert may state an opinion—and give the reasons for it—without first testifying to the underlying facts or data. But the expert may be required to disclose those facts or data on cross-examination.
(b) Voir Dire Examination of an Expert About the Underlying Facts or Data. Before an expert states an opinion or discloses the underlying facts or data, an adverse party in a civil case may—or in a criminal case must—be permitted to examine the expert about the underlying facts or data. This examination must take place outside the jury’s hearing.
(c) Admissibility of Opinion. An expert’s opinion is inadmissible if the underlying facts or data do not provide a sufficient basis for the opinion.
(d) When Otherwise Inadmissible Underlying Facts or Data May Be Disclosed; Instructing the Jury. If the underlying facts or data would otherwise be inadmissible, the proponent of the opinion may not disclose them to the jury if their probative value in helping the jury evaluate the opinion is outweighed by their prejudicial effect. If the court allows the proponent to disclose those facts or data the court must, upon timely request, restrict the evidence to its proper scope and instruct the jury accordingly.

Rule 602 points to the other type of “proper” knowledge upon which a witness may base their testimony: expertise pursuant to Rule 702. An expert witness is similar to a lay witness in that there must be a proper basis for their knowledge. For a lay witness, that basis is personal experience with some fact or event as we discussed in the previous section; for the expert witness, that basis is their specialized knowledge, training or education. There are two types of expert witnesses: The first deals with "hard" science expertise in such fields as DNA and fingerprints, and will be discussed in Kelly. The second deals with "soft" sciences where the expertise is based primarily upon experience and training as opposed to scientific method, and will be discussed in Coble. Once the expert is properly qualified, Kelly and Coble give us the standards for determining whether the opinion they offer is reliable.